There is an increasing demand for communication devices capable of operating across a variety of different frequency bands. In particular, there is an increasing demand for cellular or mobile telephones that can operate in multiple frequency bands. In such devices, separate transmit and receive filters are in general employed for each transmit and receive frequency band. In practice, bulk acoustic wave (BAW) filters, surface acoustic wave (SAW) filters, and thin film bulk acoustic resonator (FBAR) filters are in general employed.
In these multi-band devices, there is a need to connect the various RF filters to an antenna. A switching multiplexer can be employed, but such arrangement can be complex and add cost. To simplify the arrangement and reduce the size and cost, a passive matching network may be employed instead of a switching multiplexer.
In the simple case of a duplexer, a single transmission line matching network is often employed. The transmit (Tx) and receive (Rx) filters are matched by series and/or shunt transmission lines, depending on the proximity of the frequency bands with respect to each other, and the topology of each filter. For example, if the bands are close in frequency (e.g., separated in frequency by 1% or less) such as for personal communications services, code division multiple access (PCS-CDMA), a 90° (quarter wavelength) series transmission line is often placed in front of the Rx filter. If the bands have a larger frequency separation, then shunt transmission lines of various lengths may also achieve good matching.
The complexity of the matching network increases with the number of filters. A common approach is encountered when the matching network is embedded with the filters into a multiplexer, for instance by the filter designer. In that case, in general the matching network is then empirically designed with transmission lines and various lumped elements. Once the matching components and filter topologies are chosen, an RF simulator can be employed to optimize the component values for best performance.
While this is a practical approach that can produce good results, it is nevertheless risky because it can use too many matching components and is not guaranteed to produce acceptable results.
FIG. 1 shows an exemplary multi-band multiplexer with RF matching network employing series and shunt transmission lines. The multiplexer of FIG. 1 is a Universal Mobile Telecommunications System/Korean Personal Communication Services (UMTS/KPCS) quadplexer (four frequency bands) for the UMTS Tx band (1920-1980 MHz), UMTS Rx band (2110-2170 MHz), KPCS Tx band (1750-1780 MHz), and KPCS Rx band (1840-1870 MHz). The filters for the arrangement of FIG. 1 are FBAR filters.
However, the arrangement of FIG. 1 has some drawbacks. The arrangement includes a large number of components. The transmission lines are long and lossy. Also, the transmission lines have to be isolated by the rest of the components. Thus, top and bottom ground planes are employed, and therefore a four-layer printed circuit board (PCB) is used, increasing the cost and complexity of the arrangement.
What is needed, therefore, is a general matching network and method of matching an antenna or other device to a plurality of BAW, SAW, and/or FBAR filters than can alleviate one or more of these shortcomings.